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Facebook is declaring early success for the new functionality on its site that lets members notify their friends what songs they're listening to online and makes it possible for notification recipients to play back those songs from within the Facebook interface.

Six weeks after Facebook launched this feature at its F8 developer conference, more than 1.5 billion song notifications have been shared on the site using participating music applications, the company said in a blog post.

"As a result, some of our biggest music developers have more than doubled their active users, while earlier-stage startups and services starting with a smaller base have seen anywhere between a 2-10x increase in active users," the blog post reads.

The first few days after the launch of this "social music" feature were a bit bumpy as some users complained that they wanted more granular controls over the notifications.

In response, Spotify -- the poster child for Facebook social music -- introduced a "private listening" mode so that people can shut off the notifications spigot when listening to music they don't want to share with their friends.

This week, Facebook reported that, since F8, Spotify has gained more than 4 million users, while online music provider MOG has seen its user base grow 246 percent. Other music apps seeing strong growth in usage are Earbits, Rdio, Slacker and Deezer, according to Facebook.

"It's still early, but these results show that the Open Graph can be a powerful discovery mechanism for users and drive significant growth for developers," the blog post reads.

Facebook is also providing similar social sharing capabilities for other media, such as news articles, movies and TV shows with partners like Yahoo, Netflix and Hulu.